Rosmarinic acid (RA) is an ester of caffeic acid and 3,4-dihydroxyphenylacetic acid. It is also a secondary metabolite of various plant species including those of Lamiaceae. Although rosmarinic acid was first extracted from rosemary (Rosmarinus officinalis L.), unlike the other abundant antioxidants compounds of rosemary (carnosic acid and carnosol), rosmarinic acid is more polar and appropriate for use in other food products. There is an interest in developing products based on the more polar rosmarinic acid that will likely have greater antioxidative efficacy in beverages, sauces, and emulsions. In addition, this molecule is known to have unique properties including antiviral, antibacterial, and anti-inflammatory activities (Mazumder A, Neamati N, Sunder S, Schulz J, Pertz H, Eich E, and Pommier Y. 1997. Curcumin analogs with altered potencies against HIV-1 integrase as probles for biochemical mechanisms of drug action. Journal of Medical Chemistry. 40:3057-3063; Szabo E, Thelen A and Paterson M. 1999. Fungal elicitor preparations and methyl jasmonate enhance rosmarinic acid accumulation in suspension cultures of Coleus Blumei. Plant Cell Reports 18: 485-489; Hooker C W, Lott W B and Harrich D. 2001. Inhibitors of human immunodeficiency virus Type 1 reverse transcriptase target distinct phases of early reverse transcription. Journal Virology. 75: 3095-3104).
Mint family species, including peppermint (Mentha piperita L.) and spearmint (Mentha spicata L.), are important specialty crops valued for the essential oils produced in trichomes on the surface of leaves. Spearmint is particularly known as a major source of carvone-rich essential oil for perfumery and flavoring industries and is grown worldwide. It is a fast-growing perennial crop that can biosynthesize significant amounts of rosmarinic acid and other phenolics. In addition, mint plants are capable of producing underground rhizomes which can be used as propagates for field planting. They are also capable of rapidly producing rooted branches which can aid in faster propagations. They are easy to cultivate, can sustain several harvests annually, and exhibits a rapid re-growth after each harvest.
While progressive genetic improvement was focused on increasing the essential oils, little or no effort has yet been made in the improvement of M. spicata for better production of rosmarinic acid or other antioxidant molecules. Today the major mint production areas of the United States, and even of the world, for the purpose of essential oils are found in the Columbia River basin, east of the mountains in Washington, Willamette Valley in Oregon, Michigan, and Indiana.
The reported levels of rosmarinic acid in M. spicata clones ranges from 7.1 to 14.3 mg/g dry weight (DW) basis (Wang H, Provan G J, Helliwell K. 2004. Determination of rosmarinic acid and caffeic acid in aromatic herbs by HPLC. Food Chemistry, 87:307-311; Kosar M, Dorman H J D, Baser K H C, Hiltunen R. 2004. Screening of free radical scavenging compounds in water extracts of Mentha samples using a postcolumn derivatization method. Journal of Agricultural and Food Chemistry 52(16): 5004-5010). However M. spicata clones biochemically selected in vitro for elevated levels of phenolics ranged from 20 to 67 mg/g DW (Fletcher R S, McAuley C and Kott L S. 2005a. Novel Mentha Spicata clones with enhanced rosmarinic acid and antioxidant activity. Proc. WOCMAP III, Vol. 6: Traditional Medicine and Nutraceuticals Ada Horticulture. 6S0, ISHS. pp 31-40 SA-08-06337; Fletcher R S, McAuley C and Kott L S. 2005b. Heat stress reduces the accumulation of rosmarinic acid and the antioxidant activity of Spearmint (Mentha spicata L.). Journal of Science of Food and Agriculture 85:2429-2436 SA-09-06343). One particular M. spicata clone, 700B (US Pat. Appl. 2010/0137433), selected in vitro, was found to produce 87 to 118 mg/g of rosmarinic acid on a DW basis. Rosmarinic acid levels are known to increase with light intensity, day length and optimal temperature (>70° F.) (Fletcher et al., 2005a). Extraction of rosmarinic acid from a hyper-accumulating spearmint variety is crucial for economically viable commercial rosmarinic acid production. Therefore developing spearmint varieties with high levels of rosmarinic acid combined with high biomass will provide an economically valuable rosmarinic acid source.